Equalizer for transmission lines



April 24, 1951 K W,l PFLEGER 2,550,595

EQUALIZER FOR TRANSMISSION LINES MANUAL AaJusrsns (r/s. s)

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EQUALIZER FOR TRANSMISSION LINES FIG; 2

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CONTROL C/RCU/TJ FOR LOSS FIG.

BAND PASS F/L /NVENTOR y K. WPI-'LEGER April 24, 1951 K. w. PFLEGER EQUALIZER FOR TRANSMISSION LINES 4 Sheets-Shea?l 3 Original Filed Nov. 18, 1947 RESO/VA TE NEAR PILOT FREO.

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EQUALIZER FOR TRANSMISSION LINES .Y GHANNELS 0F FREO. Ff-* y LOW PASS FILTERS ATTORNEY Patented Apr. 24, 1951 EQUALIZER FOR TRANSMISSION LNES Kenneth W. Pfleger, Arlington, N. J., assigner to Bell lTelephone Laboratories, Incorporated, New York, N. Y., a corporation of New .York

Original application November 18, 1947, Serial No. 786,745. Divided and this application July 19, 1949, Serial No. 105,517

6 Claims.

This invention relates to electric circuits and more particularly to circuits for equalization of waves transmitted over transmission lines or other'media. This application is a division of application Serial No. 736,745, iiled November 18, 1947.

It is an object of this invention to improve the equalization of wide band signals and especially of those signals transmitted overa path which is so longr electrically as to cause phase shifts between components of diierent frequencies.

The transmission or television signals involves manyproblems inconsequence of the relatively wide band of frequencies over which the signals are spread. Not `the `least of these problems is that of equalizing or compensating for phase variations caused, -for example, by temperature changes on transmission line circuits carrying these wide band signals. It has previously been proposed to provide basic compensation for such phase variations by the use of equalizing networks which introduce iixed envelope delay (ri/dw) into the system and to further compensate for these variations 4by providing adjustable delay equalizers in conjunction with the fixed delay equalizers. `After all this equalization, on long broadband cable circuits there still might remain small variable humps in the loss versus frequency and envelope delay versusY frequency characteristics. what is called mop-up equalization, that is, equalization of these small variable humps remaining after the usual equalization has been made. Y

In a system disclosed in Patent 2,379,744, issued July 3, i945, to K. W, Pfleger, auxiliary signals or waves transmitted over pilot channels (three are given by way of example) are utilized to operate continuously adjustable networks consisting of stationary impedance elements. The impedance of each of the adjustable elements is changed by means of a current of varying intensity caused to be varied in accordance with the phase diierence of two waves of the same frequency (F) obtained respectively `by demodulating two modulated Waves transmitted over the television line, each comprising a pilot frequency (a different one for each one of the waves) modulated by the fixed frequency F0 which is lower. than either of the pilot frequencies. They two demodulated waves of the frequency Fo are applied to a push-pull phase detector after having been initially adjusted by phase Shifters to be in quadrature. As long as this phase relation is maintained, the output ofthe phase detector is zero,` regardless of The present invention relates Vto level changes on the television line. When the phase relation between the two demodulated Wave changes due to variationsin delay over the line, the balance is destroyed and the output current of the phase detector has an appreciable magnitude in one direction or the other. When the phase relation between the input currents for the phase detector changes' in the opposite direction, the direction of output current is reversed. These output currents are, as pointed out above, utilized to control the impedancesof variable elements in'an adjustable equalizer. A

rst pilot frequency F1 is located at or slightly below the lower' edge of the television band', a second pilot frequency F2 vis located about in the center of the band (preferably in a dead portion of the television band) ,and a third at or slightly above the lupper edge of the television band. The delay distortion over the frequency range F1 to F2 is corrected with the aid of one of the phase detectors while the distortion over the frequency range F2 to Fs is corrected with the aid of the second phase detector. In the system described in the above-mentioned Pieger patent, the side--A bands are transmitted overthe cablefo'r other path and utilized with thepilot frequency waves at the yreceiving end to produce waves at the envelope frequency which waves are applied to the phase detectors.

' In a mop-up equalizer utilizing an adjustable attenuator or equalizer in accordance with the present invention (the mop-up equalizer in one of its aspects being a special case of and an extension of the patented Plieger arrangement),v both automatic delay distortion and automatic attenuation distortion are compensated by the same pilot frequencies, of which a great number are provided, they preferably being located throughout the television band at intervals cor#V responding tothe line scanning frequency of the television image. Due to the relatively close spacing of the pilot frequencies, only the latter need be transmitted and the step of modulation (in the above-identied patented arrangement) is not necessary. The pilot frequencies are utilized at the receiving station to vary the magnitudes of humps o1 loss which are inserted into the system at the proper frequencies to give substantially ilat compensation of the' attenuation versus frequency and Venvelope delay versus frequency characteristics.

In accordance with the present invention, there is provided an attenuator or equalizer comprising a first section, consisting of a multiplicity of parallel-connected branches each containing` a tuned trici;

Y even-numbered pilot frequencies.

circuit resonating at a different one of a multiplicity of odd-numbered pilot frequencies and a thermistor, followed, after a ressitance pad, by a section like the first section but tuned to the Varying the resistance of a particular thermistor varies the magnitude of the hump of loss inserted into the lsystem at the particular pilot frequency. The resultant characteristic of the two sections is substantially fiat for the frequency range of interest.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof in which:

Fig. 1 is a schematic block diagram of a p0rtion of the sending end apparatus of a television system including the sending end equipment of a mop-up equalizer embodying an adjustable attenuator or equalizer' in accordance with the invention;

Fig. 2 is a schematic block diagram of receiving end apparatus adapted to be utilized with the sending end equipment shown in Fig. 1;

Fig. 3 is a circuit diagram of one of the manual level and phase adjusters forming part of the equipment shown in Fig. 1 and Fig. 2;

Fig. 4 is a circuit diagram of one of the control circuits forming part of the arrangement of Fig. 2;

Fig. 5 is a circuit diagram of an adjustable attenuator or equalizer in accordance with the invention and which is suitable for use in the arrangement of Fig. 2;

Fig. 6 is a graphical representation to aid in understanding the invention;

Fig. '7 is a schematic block diagram of control circuits which can be utilized in the arrangement of Fig. 2; and

Fig. 8 is a circuit diagram of a phase detector which can be used in the control circuits of Fig. 7

Referring more specifically to the drawings, Fig. shows an exemplary embodiment of an adjustable attenuator or equalizer in accordance with the invention. in order to illustrate the manner and use of such an equalizer, however, reference will rst be made to Figs. 1 and 2 which show, in block diagram form, sending and receiving end portions ID and II, respectively, of a television system in which mop-up equalization of loss and envelope delay are employed. For simplicity in the drawings, details of the equipi ment for generating, transmitting and utilizing the video signal at the receiving station to produce an image of the object have not been shown since the present invention is concerned primarily with the loss and delay equalization of the television signals.

Referring first to the sending end equipment IU shown in Fig. 1, a standard frequency oscillator I2 generates oscillations of, for example, 1000 cycles per second, and this generated wave is applied through a hybrid coil I3 to a multivibrator and harmonic generator It of any suitable form to produce a group of accurate frequency waves. (The. standard frequency wave can also be applied through the filter 36 to the synchronizing circuits forming part of the television sending set l5, for control purposes.) By means of a multiplicity (for example there might be 191 for television having 441 lines and 30 frames per second) of narrow band-pass filters 20, 2l, 22, 23, 25, 25 29, 3D tuned to frequencies separated by a frequency of 13.23 kilocycles (the line scanning frequency), a multiplicity of frequencies are selected as follows: Fc (carrier frequency-for example, 300 kilocycles per second), Fc-F (where F is one-half line `scamfiing frequency), Fc-l-F, Fc-3F, Fc-tF,

Fc-H2N*3)F. Where the lower sideband of the television signal is suppressed, it is necessary to transmit relatively few pilot frequencies below Fc. The carrier frequency Fc is modulated in the modulator l5 with a video signal passing the hybrid coil I3 from the television sending set l5 and the resultant modulated wave is passed through a single sideband filter I'I and the amplifier E3 to the toll line I9. The selected frequencies Fc-F up to Fc-I-(2N-3)F are each passed through an individual one of the manually adjusted phase and level adjusters SI, 32, 33, 34, 35 39, rit (to compensate for any phase or levci change in the filters). Suitable phase and level adjusters are shown in Fig. 3 and will be described below. Each of the selected frequencies is then passed through an individual one of the narrow band-pass filters M, 42, A3, tf1, 49, 5E? to prevent that frequency from interacting with the phase or level adjuster for any of the other frequencies. Then all of the selected frequencies are applied through the ampliiier i8 to the toll line i9 and transmitted to the receiving station I I shown in Fig. 2.

At the receiving station il of Fig. 2, all of the selected (pilot) frequencies (Fc-F to inclusive) as well as the transmitted modulated wave containing the video signals are passed through an amplifier 5i and then through a delay distortion equalizer 52 (which will be described more fully below in connection with Fig. 5) and another amplifier 53. The output current of the amplifier 53 is divided into three parts as follows: (1) a portion going througha resistance pad 5t to an output circuit terminal 55 to which are applied (by means to be described below) the pilot frequencies to oppose those in the amplier 53 and thus leave only the transmitted modulated wave containing the video signal, (2) a portion going through a feedback circuit containing variable attenuation equalizers 55, and (3) a portion going through a multiplicity of parallel circuits each comprising one of the narrow band-pass filters 6I, 52, 63, 64, 65 65,1 and one of the amplifiers'i, 12, '13, '14, 'I5 79, 85. The output current from each of the lastmentioned amplifiers is then divided into three parts, (1) a portion (A) going through anindividual one of the control circuits 5l for the loss adjuster (which circuits will be described more fully below in connection with Fig. 4) which varies the current through an individual one of the thermistor heaters 8l, 82, 83, 85, 85 89, 5c in one of the attenuation equalizers of the feedback circuit 55, (2) a portion (B) going through individual phase and level adjusters 58 (which will be described more fully below in connection with Fig. 3) and narrow band-pass lters SI, 52, 53, 5Fl, 95 9S, lill] to the output circuit terminal whereby the pilot frequencies are adjusted in phase and amplitude to oppose those in the output circuit of amplifier 53, and (E) a portion (C) going to an individual one of the control circuits 59 (which will be described more fully below in connection with Fig. 7) for varying in each case the power of an individual heater for an individual impedance member of Referring now to Fig. 5, this figure shows one form of equalizer which, in accordance with the invention, can be used as the delay distortion equalizer 52 or the attenuation equalizer 56. It comprises a first section IOI consisting of a multiplicity of 4parallel-connected branches HI, H3,

H5, H1 and H9, Bn respectively resonating at a different one of the odd-numbered pilot frequencies and a thermistor, followed after resistance pad |62 by a section |53 like the rst section IBI but tuned at the even-numbered pilot frequencies. Considering one of said branches, as, for example, the branch III, it comprises a capacitor member IM, an inductance member |05, resistance member |05' and a thermistor member |01, the other parallel branches I |3, I I5, I I1, I I9, Bn, and also those in the even-numbered section |03 being similar to the branch I H except that the frequency of the tuned circuit varies. By varying the resistance of a particular thermistor |01 by varying the current in its corresponding heater 8|, 83, 85, 51, 8S, Hu, the magnitude of the hump of loss inserted into the system at that pilot frequency can be varied. The resultant of the two sections IUI and |53 is substantially flat for the over-all frequency range. As the arrangement shown in Fig. 5 produces both attenuation and delay it can be used either as a delay distortion equalizer 52 or attenuation equalizer 55. i

When the thermistors llare all at some average value, it is desirable that the receiving circuit of Fig. 2 shall have an over-all flat characteristic. `Since the network of Fig. 5 produces peaks of loss (or attenuation) at each odd-numbered resonant frequency, it is necessary to have in connection therewith a similar circuit |53 having the parallel branches thereof tunedto evennumbered pilot frequencies. At average values of the thermistor, the loss hump has'about the same peak value at all pilot frequencies and the loss characteristic due to the one network is complementary to that of the other so that over-all transmission is flat when all the thermistors are at an average value of resistance. The use of sinusoidal humps is advantageous. In general, the average values of thermistors or other possible variables, the odd-numbered network Ill! produces an over-all loss, for example, as shown by the dash line curve in Fig. 6. The even-numbered network |03 gives the complementary loss shown by the full line curve so that the resultant is at for the frequency range of interest. tions from the average of one or more networks, the resultant can be made to have either narrow or broad humps depending on the changes occurring in adjacent channels. yOn Fig. 6, which is a plot of loss versus frequency, the loss curves for both halves of the equalizer and the resultant have been shifted to refer them to an arbitrary zero loss axis in order to facilitate comparison. A curve similar to Fig. 6 can be drawn for delay versus frequency.

When simple networks of the type shown in Fig. 5 are used for loss equalizers, delay humps are also produ-ced and whenthey are used for delay equalizers, loss humps are also produced Resonant shunts are 'shown in Fig. 5 merely to illustrate how the pilot channels can control humps of loss or delay.

i In the circuit of Fig. 2 the delay distortion equalizer 52 (such as that shown in Fig.r5) is shown at the input to the amplifier 53 in'order to prevent unfavorable phase. relations from arising ,in the feedback circuit includinglthe attenuation equalizer 56. It is intended that small incidentalv loss variations caused by the delay network be automatically compensated by the attenuation equalizer and incidental delay variations of the latter, if not too great, be compensated for by the delay equalizers. By providing practically instantaneous controls using fast-acting thermistors in the variable networks, the two types of compensation can be made to cooperate simultaneously in producing over-all ilat loss and delay versus frequency characteristics.

Each of the control circuits 51 in branch A of the circuit of Fig. 2 can be of any suitable form. An example of one satisfactory circuit is shown in Fig. 4. In the arrangement of Fig. 4, each pilot frequency is applied to a rectifier |6| through a high frequency attenuator |62 having constant impedance. The output of the rectifier appearing By variaacross the resistor |53 is applied to any convenient form of oscillator |64 to control the variable output to heat any one of the thermistors 8|, 82, 83 90. The connections in Fig. 4 should be poled so that a sudden increase in level of a pilot channel on the line causes the corresponding thermistor resistance to vary in a direction to increase the amount of negative feedback (through the attenuation equalizer 56) permitted at this frequency, until the level of the pilot frequency at the amplifier output is reduced to the former value. A similar procedure follows with opposite signs when the pilot level decreases.

The branch B of circuit shown in Fig. 2 includes a multiplicity of phase and level adjusters 58. A suitable manual level and phase adjuster is shown in Fig. 3. This comprises a high fre-` quency attenuator |66, a shunt-connected inductance member |61 and variable capacitor member |68. The capacity |68 can be varied to produce the level and phase adjustment desired.

Reference will now be made to Fig. '7 which is a single line schematic diagram of a control circuit suitable for use in the path C of the circuit shown in Fig. 2, or in other words, it can be used as the control circuit 59 of Fig. 2., Before describing the circuit arrangement of Fig. 7 it seem-s best y to describe its function. Supposev the circuit has been lined up initially to have zero delay distortion. Suddenly the phases of the N pilot channels are shifted respectively by increments i, c2, [33,

etc., where the subscript in each case denotes the number of the channel. It is assumed satisfactory if the automatic delay distortion compensator so operates that 2-1==a-z=43, etc. because, with equally spaced pilot frequencies, this keeps Ali/Aw a constant over the entire transmitted frequency range, where to represent delay instead of is assumed satisfactory so long as wiggles in the line characteristic have a periodicity considerably greater than 2F. lIt is desirable that changes introduced by the delay adjuster 52 shall be gradual between pilot frequencies so that no iarp irregularities are introduced. 'it happens that when any two frequencies f1 and f2 beat together in a demodulator and the difference frequency is considered, any sudden phase incre- 33, lil/i, itt, itl' ESE, it, and at the outputs oi all of these ilters the diiierence frequency, In each channel, the

phase shift is respec ,'vely equai (within a constant) to ps2-,61, [i3-oa i-s, In order to test these phase increments for equality they are compared to some standard, for example, to [34*[33 appearingf at the output of iow-pass filter ISS, or in similar' fashion, any other lter output can be selected. An accurate means of detectingphase shift is the SiS-degree, push-pull detector shown in S. A phase detector such as that shown in Fig. 8 is sensitive to .0l degree even when level changes occur in the two inputs oi niagniude about onehalf declbe For the frequency in -erval 2in1-13,230 cycles, envelope delay sensitivity is .0G21 m-icrosecoiid. In order to compare phase changes at the output oi lowpass filter |83 with phase chanees in the output of the other low-pass filters, N-2 phase detectors ISI, 192, I, ld, ISE, i Z'i are arranged as shown in Fig. 7, each deriving one input from low-pass filter E53 and 'the other input from some other low-,lass lter lii, 82, |86, |85, iSd, ISI ISS or ist). In tandem with these connections are connected N-l manual phase level adjusters 2M, 2M, 2%, 2636, ZB, Ei, 25.31* Zi, 2id so set that when no delay distortion es` on the li e the two inputs ci' cach phase detector are o -ees and the di 'ect-current output is then Zeo. (It is possible to omit the phase adjuster ii the others have sufficient range.)

The phase detector (ii-3?. for example) shown in Fig. 8 comprises a resistance bridge 2| I acrossJ the upper and lower corners oi which is applied one wave of frequency from transformer 2|? connected to one phase and level adjuster (2&5 to 2li!) of Fig. 'l and across the right and left corners of which applied a war or" requen 2F from transform-er 2 connected to another one of the phase and level ad, -s 252| to 2 he upper and lower corners are connected retpectively to rectidrs 2|@ and 1 Between positive terminals oi these rectii'lers is connected a series circuit com rising resistors eilt and 2|? the common terminal 'IS of which is connected to the r :at-hand corner or" the bridge 2| I. ally connected resistors kZIS and 2li are shunted by a condenser 2 I 9. The operation of circuits like that shown in Fig. is v/ell known and will oniy be given a brief description here.

rihe current in the upper right arm of the bridge 2| i is the vector sum oi the two sinusoidal waves from so rees 2F@ and 2M, and the current in the lower iight of the bridge 2H is the vector diierencc of these waves. When the 'iitudes ci the two inputs equal, the vector sul imay he expressed by ES=2A cos;

and the vector difference by where A equals the magnitude constant of each component and 0 equals the phase angle between them. Now the voltages impressed on rectiers 2|Q and 2|5 are respectively proportional to the values Es and En, as are the rectied waves flowing in resistors EI and 2|?. A low-pass lter consisting; or shunt condenser 2|i and series inductance 222 removes alternating components from the output wave owing over conductors 22|. Due to the differential combination of the voltages across Elf and 2|? the open circuit output voltage across conductor 22| is proportional to (am) y 2 2 tional to 0 COS SII] 2* Returning again to the control circuit of Fig. '7, it will be noted that when s-24-3, there is a direct-current component in the output from phase detector |92 superimposed upon the current from battery 220, so that the magnitude and direction of the superimposed component are respectively indications of the magnitude and sign of the inequality of the two phase increments. Consequently the resultant direct current in leads 22| serves as an indication of the amount of the required variable delay for compensation in the frequency region between Fc-l-F and Fc-l-SF in order to shift phases so that iinally s-2=4-s. For example, suppose it is desired to increase [i3-[32. A phase hump similar in shape to the attenuation hump in Fig. 6 with a peak at Fc-l-SF shown by the dashed curve can be inserted by some suitable network. In terms of envelope delay this phase characteristic would have a delay hump at about Fc-l-ZF. It is thus evident that the various phase detectors (191 to 200) are to control delay humps having peak values about midway between the associated pilot frequencies. Fig. 6 can be assumed to apply to desirable delay equalizers at average conditions if the humps are taken to denote phase shift instead of loss. It is obvious that as the device of this sort makes s-zzi-s, it also holds etc. The outputs of the phase detectors ISI to 20 can be used to control thermistors in the delay distortion equalizer 52 shown in Fig. 5.

In lining up and adjusting the apparatus, the relative phases of the power channel should be definitely adjusted at the sending end in such a manner as not to cause serious resultant peak values on the line. The sending arrangements should be stable so that this phase relationship remains xed within about 101 degree in each case in order not to produce appreciable subsequent errors in regulation. For the same reason, the output levels of the pilot frequencies should remain constant within about i.05 decibel. Consequently, temperature control of all iilters and phase Shifters is desirable and also stabilization of the harmonic generator I4. It is desirable to give all sending circuits the same adjustment throughout the plant so that they are readily in- Aof the delay and loss adjusters give over-all flat characteristics. It is also important for certain parts of the receiving circuit to be kept at constant temperature and to be as stable as the sending circuits. After adjusting a receiving circuit it can be removed to its nal destination. Each section of circuit to be controlled by the automatic device is first given as flat an adjustment of loss and delay as possible with the basic equalizers of the system before the automatic device is added. Consequently, straightaway loss and delay distortion measurements are desired. The narrow band-pass filters which separate the pilot channels from the television current should have high impedance to prevent unfavorable shunt loss and delay of the through transmission.

It is obvious that the amount of apparatus for one sending and one receiving circuit as shown in Figs. 1 and 2 is considerable. Reference is made to the parent application, Serial No. 786,- 745, for a list of this apparatus and for a description of a switching arrangement for reducing the amount of equipment required. Another form of equalizer which can be used in the system of Figs. 1 and 2 is described and claimed in a copending application Serial No. 105,518, filed July 19, 1949.

Obviously, various changes can be made in the equalizer or attenuator described above without departing from the spirit of the invention.

What is claimed is:

1. An attenuation equalizer comprising a pair of input terminals and a pair of output terminals, a plurality of parallel circuits connected across said input terminals and also across said output terminals, each circuit comprising a serially connected condenser, inductance, resistance and a thermistor, the condenser and inductance in one path being tuned to a different frequency from that of every other path, and a heater associated with each thermistor whereby by varying the current in said heater the impedance of said thermistor and hence of the corresponding circuit is varied in accordance with signals applied to said heater.

2. An automatic delay distortion equalizer comprising a pair of input terminals and a. pair of output terminals, a plurality of parallel circuits connected across said input terminals and also across said output terminals, each circuit comprising a serially connected condenser, inductance, resistance and a thermistor, the condenser and inductance in one path being tuned to a different frequency from that' of every other path, and a heater associated with each thermistor whereby by varying the current in said heater the impedance of said thermistor and hence of the corresponding circuit is varied in accordance With signals applied to said heater.

3. An attenuation equalizer comprising a pair of input terminals and a pair of output terminals, a plurality of parallel circuits connected across said input terminals andralso across said output terminals, each circuit comprising a serially connected condenser, inductance, resistance and a thermistor, the condenser and inductance in one path being tuned to a different frequency from that of every other path, and a heater associated with each thermistor whereby by varying the current in said heater the impedance of said thermistor and hence of the corresponding circuit is varied in accordance with signals applied to said heater, adjacent ones of the various frequencies at which said tuned circuits are respectively resonant being separated by a frequency band of constant width.

4. An automatic delay distortion equalizer comprising a pair of input terminals and a pair of output terminals, a plurality of parallel circuits connected across said input terminals and also across said output terminals, each circuit comprising a serially connected condenser, inductance, resistance and a thermistor, the condenser and inductance in one path being tuned to a different frequency from that of every other path, and a heater associated with each thermister whereby by varying the current in said heater the impedance of said thermistor and hence of the corresponding circuit is varied in accordance with signals applied to said heater, adjacent ones of the various frequencies at which said tuned circuits are resonant being separated -by a frequency band of constant width.

5. An attentuation equalizer as in claim 1 in which each path is tuned to a different oddnumbered frequency in further combination with an attenuation equalizer as in claim 1 in which each path is tuned to a different even-numbered frequency.

6. An automatic delay distortion equalizer as in claim 2 in which each path is tuned to a different odd-numbered frequency in further combination with an automatic delay distortion equalizer as in claim 2 in which each path is tuned to a different even-numbered frequency.

KENNETH W. PFLEGER.

REFERENCES CITED Name Date Moulln June 18, 1935 Number 2,055,236 

